Intravascular clotting is a common disorder. One of the most common of such disorders is the formation of thrombi, or clots which form in a blood vessel or heart cavity and remain at the point of formation. Thrombi can have serious adverse effects on an individual. For example, thrombus formation in the heart can restrict blood flow, resulting in myocardial infarction (death of the heart muscle), which is one of the most severe forms of heart attacks.
In addition to having adverse effects at the point at which it forms, all or part of a thrombus can dislodge from its point of attachment and move through blood vessels, until it reaches a point where passage is restricted and it can no longer move. The sudden blockage of blood flow which results is referred to as a thromboembolism. The lungs are particularly susceptible to emboli formation because it is in the lungs where main arteries first divide into smaller arteries and capillaries after the heart has received blood flow from the venous system. Emboli trapped in the lungs interfere with gas exchange and circulation. Accordingly, methods which prevent thrombi formation are of great medical importance.
Although the process of thrombus formation is only incompletely understood, two major stages have been identified: the aggregation of platelets at the site of a blood vessel injury, and the formation of a cross-linked fibrin polymer which binds the developing clot together.
The dinucleotide, diadenosine 5',5'"-p.sup.1, p.sup.4 -tetraphosphate (AP.sub.4 A) (Formula I), an ubiquitous component of living cells, is stored in high concentrations in the dense granules of blood platelets Zamecnik, P. C. and Stephenson, M. L., Regulatory mechanisms for protein synthesis. In: Mammalian Cells, San Pietro, A., Lamborg, M. R. and Kenney, P. C. (eds.), Academic Press, New York, pp. 3-16 (1968). AP.sub.4 A is present in normal human platelets in a concentration higher than that present in any other cellular compartment. Flodgaard, M. and Klenow, M. Biochemical Journal, 208:737-742 (1983). The stored AP.sub.4 A was thought to be metabolically inert because incubation of platelets with .sup.3 H-adenosine results in labeled ATP but not labeled AP.sub.4 A. Thrombin treatment of platelets induces the complete release of AP.sub.4 A, along with other storage pool nucleotides, including ADP and the dinucleotide, diadenosine 5',5'"-p.sup.1, p.sup.3 -triphosphate (AP.sub.3 A). Luthje, J. and Ogilvie, A. Biochem. Biophys. Res. Comm. 115:253-260 (1983). AP.sub.3 A is hydrolysed in plasma to AMP (adenosine monophosphate) and ADP (adenosine diphosphate); AP.sub.4 A is degraded to AMP and ATP (adenosine triphosphate) Luthje, J. and Ogilvie. A. European Journal of Biochemistry, 149:119-127 (1985).
The precise physiological role of AP.sub.4 A has not been defined, but it has been associated with a variety of cellular metabolic events. Zamecnik, P. Anals of Biochemistry, 134:1-10 (1983). The unusually high concentration of AP.sub.4 A in platelets has led to speculation that it has a role in platelet physiology. Platelets stimulated to undergo aggregation show a second phase of aggregation upon the release of endogenous ADP stored in the dense granules. In vitro experiments have demonstrated that AP.sub.4 A competitively inhibits ADP induced platelet aggregation, causing an immediate dispersion of aggregated platelets, even when aggregation has progressed to 60% completion. Chao, F. C. and Zamecnik, P., Hoppe Seyler's Z. Physiol. Chem., 365:610 (1984). By contrast, AP.sub.3 A causes a gradual aggregation of platelets, most likely through its degradation product, ADP. The aggregating activity of AP.sub.3 A is immediately reversible upon the addition of AP.sub.4 A. Luthje, J. and Ogilvie, A. Biochem. Biophys. Res. Comm., 118:704-709 (1984).